Rewind the timeline for SVG-in-<img> when resetAnimation() is called

Attempt to mimic the behavior of animated raster images (BitmapImage)
more closely wrt when the timeline is started and reset/rewound.

BUG=177277

Review URL: https://codereview.chromium.org/206033005

git-svn-id: svn://svn.chromium.org/blink/trunk@169694 bbb929c8-8fbe-4397-9dbb-9b2b20218538